INSTRUMENTATION QUESTION AND ANSWERS PART-2

Q1: Name the test equipments used to calibrate Pressure Instruments.

Ans:    a) Manometers (For low Pressure Inst)

            b) Pneumatic Calibrators (e.g Wallace & Tiernan)

            c) Dead Weight Testers

            d) Hand Pumps with Standard Pressure Gauge

 Q2: Describe step by step how you will assemble the parts of Control Valve?

Ans: a) Carefully fix the Seat, Cage and Plug in the Valve Body

         b) Put required gaskets and fix the Bonnet tightly

         c) Fill the gland packing, use grease for lubrication and fix gland plate

         d) Fix the actuator and couple the stem with actuator (make sure valve is full closed and proper spring tension is adjusted)

         e) Fix feedback links, Valve positioner and Air regulator.

         f) Stroke check the valve and make adjustment for final calibration

         g) Leak test the valve in full close position.

Q3: Explain how the Float Level Switch works?

Ans: Liquid Fills in to the Level Switch Chamber, Which makes the float to move, the Float is linked with Iron core which moves in the head assembly inside the tube and magnetic module with micro switch assembly is fixed externally on the tube gets actuated as the Iron cores moves in and comes closer to magnetic module. The micro switch contacts get changed and gives signal to control room.

Q4: What are the advantages of Valve Positioner?

Ans: a)  Split range calibration.

         b)  Boosting up output signal for quick action.

c)      Precise control due to feedback signal.

d)     Reversing Action of Control Valves.

Q5: What are the common parts in pressure gauge.

Ans: a) Bourdon Tube, Linkage, Sector and Pinion, Pointer and Dial.

Q6: What is Span mean of the instrument mean?

Ans: It is the valve of the range of the instrument. E.g if the range of instrument is 50~200 deg C than the span is 150 deg C. So max range – min range = span.

Q7: What is function of Air Filter Regulators used on all pneumatic instruments?

Ans: As the name itself says, It filters out the small particles in the air and restricts moisture from reaching the instruments. The instruments need certain amount of pressure to function and that required pressure is adjusted by the regulator.

Q8: What is meant by I/P converter. Explain briefly where and why it is used?

Ans: It is basically a transducer which converts one form of signal into another form I/P converts current signal 4~20 milli amp. To pneumatic signal 3~15 psi. It is widely used on the control signal the valves positioners, since we receive signal from control room is electrical signal (Current ‘I’) we need to convert that signal into air pressure (P) as the control valves are operated on pneumatic system.

Q9: Name commonly used Temperature Measuring Elements?

Ans: 1) Bimetallic Strips used in Dial thermometers

         2) Filled Thermal System  -  Capillary type Temp. Sensors

         3) Thermocouples

         4) Resistance Temperature Detectors (RTD’s)

         5) Thermistors                                                

           

ELECTRONICS

Que. :  What is a diode?

Ans. :  A diode consists of two electrodes (1) Anode (2) Cathode. The current flow is only in one direction.

A diode is the most basic solid state (semi conductor) device. The above figure shows a  P.N. junction. The P. material has holes and the N. material has electrons.

FORWARD BIAS :

Where the applied voltage overcomes the barrier potential (the p side is more positive than the n side) the current produce is large because majority carriers cross the junction in large numbers. This condition is called forward bias.

REVERSE BIAS :

When the applied voltage aids the barrier potential (n side  +  ve than p side) the current in small. This state is known as Reverse Bias.

Que. :  What is a filter used for ?

Ans. :  The half wave and full wave signals are pulsating D . C . voltages. The use of such voltages is limited to charging batteries, running D. C. motors, and a few other applications. What we really have is a D.C. voltage that is constant in valve, similar to the voltage from a battery. To get a constant voltage from this, we can use a capacitor input filter.

Que. :  What is Zener Diode ? What is a voltage Regulator ?

Ans. :  The breakdown region of a p n diode can be made very sharp and almost vertical Diodes with almost vertical breakdown region are known as Zener Diodes.

A Zener diodes operating in the breakdown region is equivalent to a battery. Because of this current through Zener diode can change but the voltage remains constant. It is this constant voltage that has made the zever diode an important device in voltage regulation.

VOLTAGE REGULATOR :

The output remains constant despite changes in input voltage due to Zever effect.

Que. :  What is transistor ? What are the different types ?

Ans. :   A transistor is a three lagged semi conductor device. Basically a transistor means (transfer - resister).

Whether the transistor is pnp or npn it resembles two diodes (back to back). The one of the left is called emitter diodes, and the one on the right is the collector diode. Since two types of charges are involved transistor : are classified as bipolar devices.

Biasing the transistor :

Emitter     - Base  - Forward Bias.

Collector  - Base  - Reverse Bias.

Que. :  What is CB, CE and CC configuration ?

Ans. :

Buffer : A device or a circuit used to isolate two pt'ne circuits or stages. The emitter follows is a example of buffer.

Alpha  = IC / IE

Beta     = IC / IB

Que. :  How will you test a transistor with a multimeter. ?

Ans. :

1. Emitter +ve of meter and Base -ve output = Low resistance

2. Emitter -ve of meter and base +ve output = High resistance.

3. Collector +ve and Base -ve output = Low.

4. Collector -ve and base +ve output = High.

Emitter  :  Collector = High Resistance.

PNP : Opposite Results.

Que. :  What is a thyristor ? What are its uses ?

Ans. :   A thyristor is a special kind of semi conductor device that uses internal feedback to produce  latching action.

Use : Used for controlling large amounts of load power in motors, heaters, lighting systems etc.

Explanation : Because of the unusual connection we have a +ve feedback also called regeneration. A change in current at any point in the loop is amplified and returned to the starting point with the same phase. For instance if the 02 base current increases, the 02 collector current increases. This force base current through 01. In turn this produces a large 01 collector current which drives the 02 base harder. This build up in currents will continue until both transistors are driven in saturation. In this case the latch acts like a closed switch.

On the other hand , if something causes the 02 base current to decrease, the 02 collector current will decrease. This reduces the 01 base current. In turn, there is less 01 collector current, which reduces the 01 base current even more. This regeneration continues until both transistors are driven into cut off. At this time the latches like a open switch. This latch will always stay in open or close position.

Que. :  What are logic gates ? Explain with truth table.

Ans. :

GATE : A gate is a logic circuit with one output and one or more inputs . An output signal occurs only for control combination of input signals.

1. OR - GATE :

SYMBOL :

TRUTH TABLE :	A	B	Y
	0	0	0
	0	1	0
	1	0	0
	1	1	1

Define : An OR Gate has one output if any or all of its input are 1's.

2. AND GATE :

Define : An AND Gate has output when all inputs are present.

SYMBOL :

TRUTH TABLE :	A	B	Y
	0	0	0
	0	1	0
	1	0	0
	1	1	1

3. NOT GATE :

A not gate is also known as an inverter. This circuit has one input and one out put . All it does is invert the input signal; if the input is high, the output is low and vice versa.

TRUTH TABLE :	Input	Output
	0	1
	1	0

Buffer : This is a non inverting gate, used to drive low impedance loads.

NOR GATE : This is an OR GATE followed by an inverter.

SYMBOL :

TRUTH TABLE :	A	B	Y
	0	0	1
	0	1	0
	1	0	0
	1	1	0

Exclusive  OR GATE	A	B	Y
	0	0	0
	0	1	1
	1	0	1
	1	1	0

NAND GATE : This is an AND GATE followed by an inverter.

TRUTH TABLE :	A	B	Y
	0	0	1
	0	1	1
	1	0	1
	1	1	0